In Wheelock and Boylan U.S. Pat. No. 3,087,790 there is disclosed a process for reductive decomposition of calcium sulfate which can produce a calcium oxide product substantially free of calcium sulfide together with recoverable sulfur dioxide. To minimize contamination of the CaO product with CaS, it is necessary to control the temperature of the reaction within narrow limits, namely within the range from 2150.degree. to 2250.degree. F., and to control the quantity of reducing gas (CO and H.sub.2) to about 1 to 7% of the gaseous atmosphere, the CO.sub.2 in the gaseous atmosphere being present in an amount greater than the combined amounts of the reducing gas and SO.sub.2. When the process is operated on a continuous basis, therefore, the rate of the reduction reaction is limited, which in turn limits the through-put rate of the calcium sulfate. As disclosed in the later Wheelock and Boylan U.S. Pat. No. 3,607,045, the reductive decomposition may be carried out in a single fluidized bed with the reducing gas produced in situ by partial combustion of a hydrocarbon gas within the fluidized bed. This arrangement permits the air and fuel gas to be preheated by indirect heat exchange contact with the off-gas from the reactor, and the calcium sulfate feed can also be preheated by direct heat exchange contact with the off-gas. Substantial fuel and heat economies result, but it is sill necessary to carefully control the temperature and the CO and H.sub.2 content of the reducing gas. U.S. Pat. No. 3,607,045 specifies the use of an amount of air providing from about 1.3 to 1.8 moles of O.sub.2 per mole of C in the fuel gas. Reaction rates and through-put rates are therefore limited.
The equations representing the reactions involved in reductive decomposition of CaSO.sub.4 by CO and H.sub.2 are: EQU CaSO.sub.4 + CO .fwdarw. CaO + CO.sub.2 + SO.sub.2 (1) EQU caSO.sub.4 + H.sub.2 .fwdarw. CaO + H.sub.2 O + SO.sub.2 (2) EQU caSO.sub.4 + 4 CO .fwdarw. CaS + 4CO.sub.2 (3) EQU caSO.sub.4 + 4 H.sub.2 .fwdarw. CaS + 4H.sub.2 O (4)
in general, the desired product reactions of equations (1) and (2) are preferentially favored over reactions (3) and (4) by mildly reducing conditions. The undesired reactions of equations (3) and (4) are promoted by more highly reducing conditions. However, it is apparent that the rate of all of the reactions can be increased by increasing the concentration of the reducing gas (CO and/or H.sub.2), the driving force of the decomposition reactions being generally proportional to the concentration of the reducing agents. But if the gas phase is strongly reducing, calcium sulfide will be produced in objectional amounts and if the gas phase is weakly reducing, the rate of desulfurization will be undesirably slow.
From the standpoint of minimizing investment capital in relation to production capacity, it would be desirable to use as large concentrations of the reducing agents as possible. On the other hand, a high quality lime (CaO) product should be as free as possible from CaS. Further, where the product is incompletely desulfurized, either because of the presence of CaSO.sub.4 or CaS, the recoverable SO.sub.2 is reduced. The improved process of the present invention provides a greatly improved means for accomplishing these conflicting objectives.
A related problem is a tendency of the calcium sulfate feed to sinter during the high temperature treatment required for the reductive conversion. With natural calcium sulfate ores, such as gypsum and anhydrite, which are relatively pure gypsum or anhydrite, sintering can be largely avoided by maintaining temperatures below 2250.degree. F. However, calcium sulfate wastes from various manufacturing operations and industrial pollution control systems have a greater tendency to sinter because of the presence of other ingredients which reduce the sintering temperature of calcium sulfate. Consequently, objectionable sintering may occur within the temperature range of 2150.degree. to 2250.degree. F., which heretofore has been believed to be the optimum temperature for converting calcium sulfate to calcium oxide with minimized formation of calcium sulfide.
Prior to the present invention, the state of the art indicated that temperatures below 2150.degree. F. would cause incomplete desulfurization and formation of calcium sulfide. With the improved process of the present invention, however, lower temperatures than 2150.degree. F. can be used without objectionable contamination of the lime product with CaS. Further, the lower temperatures permit the conversion of calcium sulfate wastes which otherwise would be subject to sintering.
Calcium sulfide can be removed from calcium oxide by a high temperature oxidizing roast. For example, as disclosed in Campbell et al U.S. Pat. No. 3,582,276, after calcium sulfate is treated with a reducing gas, calcium sulfide in admixture with calcium oxide can be treated with air or other oxygen-containing gas to convert the CaS to CaO and SO.sub.2. The equation can be represented as follows: EQU CaS + 3/2 O.sub.2 .fwdarw. CaO + SO.sub.2 ( 5)
however, calcium sulfide can oxidize to calcium sulfate by the following reaction: EQU CaS + 2 O.sub.2 .fwdarw. CaSO.sub.4 ( 6)
while the reaction of equation 6 eliminates the undesirable CaS, the reformation of CaSO.sub.4 results in incomplete desulfurization of the product with decreased production of the SO.sub.2 by-product.
In the early 1950's, experiments relating to the reductive decomposition of calcium sulfate were conducted by Walter M. Bollen, as a graduate student in the Department of Chemical Engineering, at Iowa State College, Ames, Iowa. The results of these studies are reported in Bollen's Ph.D. Thesis, "Thermal Decomposition of Calcium Sulfate" (1954). These experiments were carried out in a batch-type fluidized bed reactor, which was supplied with the products of combustion of burning natural gas and air in a combustion chamber located outside of the fluidized bed. Bollen experimented with oxidation treatments to remove CaS from the CaO product, as reported in his above cited 1954 thesis, including the use of subsequent oxidizing roasts for reducing CaS in a CaO product. A similar post-treatment procedure is disclosed in the U.S. Patent of Campbell et al, No. 3,582,276. More specifically, at the conclusion of the reductive decomposition, Bollen either increased the ratio of air to gas much above the theoretical amount for complete combustion of the fuel gas, or the fuel gas was turned off entirely and only air was supplied to the fluidized bed. In either case, the result was a subsequent or last stage oxidizing treatment of the batch of CaO product to remove contaminating CaS.
Bollen also concluded that the CaS content of the CaO product could be reduced by decomposing the calcium sulfate at his recommended temperature of about 2350.degree. F. with an air-gas ratio representing substantially 100% stoichiometric air for complete combustion as determined without reference to the CaSO.sub.4 decomposition. Theoretically under this condition, the fluidizing preformed combustion gas entering the bed should contain no reducing gases (CO and/or H.sub.2). However, Bollen's data appears to indicate that under the condition assumed to be 100% stoichiometric air for complete precombustion, traces of both O.sub.2 and CO were present; analysis of the combustion gas as introduced showing amounts of carbon monoxide and oxygen, such as 0.5% CO and 0.7% O.sub.2. At 95% precombustion stoichiometric air, the gas analysis showed no oxygen and more carbon monoxide (viz. 2.6% CO; 0.0% O.sub.2). However, at 105% precombustion stoichiometric air, the gas analysis showed no carbon monoxide and more oxygen (viz. 0.0% CO; 1.4% O.sub.2). This data is difficult to interpret.
Bollen hypothesized that at 100% precombustion stoichiometric air, alternating reducing and oxidizing conditions may have been obtained in the reactor. His observations overlook the O.sub.2 formed by CaSO.sub.4 decomposition, which would provide a total excess of O.sub.2. Moreover, since the driving force of the reaction of carbon monoxide with oxygen to produce carbon dioxide is exceedingly large at the temperatures employed in Bollen's reactor, if both reducing and oxidizing conditions occurred, a more likely explanation is that the relative proportions of fuel gas and air fluctuated. With slight variations from 100% theoretical precombustion stoichiometric air over gas sampling intervals of several minutes, the collected gas sample might possibly contain small amounts of both carbon monoxide and oxygen, the sample, in effect, being an average of the slightly varying gas conditions in the reactor over a finite period of time. Whatever the theoretical explanation, it is apparent that Bollen's optimum conditions are not feasible for commercial decomposition of calcium sulfate. Even assuming that both carbon monoxide and oxygen can be present when the reactor is supplied with a gas-air combustion gases the amount of CO (and/or H.sub.2 ) would be too small for effective reductive conversion of calcium sulfate to calcium oxide; that is, the reaction rate would be too slow for commercial use. Further, desulfurization would be incomplete in reasonable reaction times, and it would be expected that the by-product SO.sub.2 would be produced in low yields at dilutions, which would make recovery impractical on a commercial basis.
The U.S. Patents of Robinson et al, Nos. 3,717,700 and 3,763,830 related to a process and apparatus for burning sulfur-containing fuels such as powdered coal. A fluidized bed of lime (CaO) is utilized to capture SO.sub.2 released from the burning coal. The lime is thereby converted to calcium sulfate, which is decomposed to lime for process reuse in a separate fluidized bed regenerator. In the lime regenerator, air is introduced into the bottom of the bed as the fluidizing gas. The powdered coal is pneumatically conveyed with air into the lower portion of the regenerator. The powdered coal provides the fuel for the regeneration. In operation, the air flow is controlled to produce an overall excess of oxygen, the off-gas from the regenerator containing from about 0.5 to 2.5% oxygen.
By the improved process of the present invention, calcium sulfate can be decomposed at a greatly accelerated rate under highly reducing conditions, without paying the price of objectionable calcium sulfide contamination of the final lime (CaO) product as discharged from the reactor. In the reductive decomposition reaction which occur in the lower portion of the bed where partial, limited in situ combustion of the fuel also takes place, the calcium sulfate is subjected to highly effective rapid rate reducing conditions. At the same time, although such strong reducing conditions favor the production of the undesirable calcium sulfide, the oxidizing conditions maintained in the upper portion of the bed expose the rapidly circulating particles to conditions converting CaS to CaO, while any reformed CaSO.sub.4 is continuously circulated through the reducing zone. More complete desulfurization is thereby achieved, and the resulting product can be substantially free of both calcium sulfate and calcium sulfide. By having the oxidizing zone above the reducing zone, the off-gas by-product can contain recoverable SO.sub.2 substantially free of sulfur vapor or reduced sulfur gases, such as H.sub.2 S, carbonyl sulfide, etc.
From the standpoint of heat conservation, the heat generated by the exothermic oxidation reactions of equations (5) and (6), as they occur in the oxidation zone, can contribute to the total heat required for the endothermic decomposition reactions of equations (1) and (2). In the reducing zone, reactions (3) and (4) are exothermic. However, the process does not depend on these reactions to supply the energy for reactions (1) and (2). Because of the continuous circulation of material within the fluidized bed reactor, the occurrence of exothermic reactions in another portion of the bed, does not lead to undue fluctuations in the bed temperature. On the contrary, the temperature throughout the fluidized bed can be controlled to relatively stable temperature, which favors the desired reactions and avoids sintering.